DESCRIPTION

This is the classic Point example. It is taken directly from the Perl 6 Apocalypse 12 document, and is similar to the example found in the classic K&R C book as well.

As with all Perl 5 classes, a Moose class is defined in a package. Moose handles turning on strict and warnings for us, so all we need to do is say use Moose, and no kittens will die.

When Moose is loaded, it exports a set of sugar functions into our package. This means that we import some functions which serve as Moose "keywords". These aren't real language keywords, they're just Perl functions exported into our package.

Moose automatically makes our package a subclass of Moose::Object. The Moose::Object class provides us with a constructor that respects our attributes, as well other features. See Moose::Object for details.

Now, onto the keywords. The first one we see here is has, which defines an instance attribute in our class:

has 'x' => (isa => 'Int', is => 'rw', required => 1);

This will create an attribute named x. The isa parameter says that we expect the value stored in this attribute to pass the type constraint for Int (1). The accessor generated for this attribute will be read-write.

The required => 1 parameter means that this attribute must be provided when a new object is created. A point object without coordinates doesn't make much sense, so we don't allow it.

We have defined our attributes; next we define our methods. In Moose, as with regular Perl 5 OO, a method is just a subroutine defined within the package:

sub clear {
my $self = shift;
$self->x(0);
$self->y(0);
}

That concludes the Point class.

Next we have a subclass of Point, Point3D. To declare our superclass, we use the Moose keyword extends:

extends 'Point';

The extends keyword works much like use base. First, it will attempt to load your class if needed. However, unlike base, the extends keyword will overwrite any previous values in your package's @ISA, where use base will push values onto the package's @ISA.

It is my opinion that the behavior of extends is more intuitive. (2).

Next we create a new attribute for Point3D called z.

has 'z' => (isa => 'Int', is => 'rw', required => 1);

This attribute is just like Point's x and y attributes.

The after keyword demonstrates a Moose feature called "method modifiers" (or "advice" for the AOP inclined):

after 'clear' => sub {
my $self = shift;
$self->z(0);
};

When clear is called on a Point3D object, our modifier method gets called as well. Unsurprisingly, the modifier is called after the real method.

In this case, the real clear method is inherited from Point. Our modifier method receives the same arguments as those passed to the modified method (just $self here).

Of course, using the after modifier is not the only way to accomplish this. This is Perl, right? You can get the same results with this code:

sub clear {
my $self = shift;
$self->SUPER::clear();
$self->z(0);
}

You could also use another Moose method modifier, override:

override 'clear' => sub {
my $self = shift;
super();
$self->z(0);
};

The override modifier allows you to use the super keyword to dispatch to the superclass's method in a very Ruby-ish style.

The choice of whether to use a method modifier, and which one to use, is often a question of style as much as functionality.

The new constructor accepts a named argument pair for each attribute defined by the class, which you can provide as a hash or hash reference. In this particular example, the attributes are required, and calling new without them will throw an error.

my $point = Point->new( x => 5 ); # no y, kaboom!

From here on, we can use $point and $point3d just as you would any other Perl 5 object. For a more detailed example of what can be done, you can refer to the t/recipes/moose_cookbook_basics_point_attributesandsubclassing.t test file.

Moose Objects are Just Hashrefs

While this all may appear rather magical, it's important to realize that Moose objects are just hash references under the hood (3). For example, you could pass $self to Data::Dumper and you'd get exactly what you'd expect.

You could even poke around inside the object's data structure, but that is strongly discouraged.

The fact that Moose objects are hashrefs means it is easy to use Moose to extend non-Moose classes, as long as they too are hash references. If you want to extend a non-hashref class, check out MooseX::InsideOut.